Obviously, the models are equivalent conceptually. There are some prominent folks in the signal integrity community that will tell you the above equations are wrong, but then they will present equations that derive the inductance of a transmission line circuit and its capacitance in terms of the geometry of the line and its return path (i.e., any nearby ground or power planes). These values are both low and can be ignored at moderate frequencies (i.e., hundreds of MHz), which reduces to the well-known equation for characteristic impedance for a lossless transmission line:Ĭharacteristic impedance of a lossless transmission line Note that R and G account of the resistance of the conductor and parasitic conductance of the substrate, respectively, both on a per unit length basis. If you use the lumped circuit model for a transmission line, you’ll find that the characteristic impedance of a transmission line is just the ratio of the per-length inductance to per-length capacitance:Ĭharacteristic impedance of a lossy transmission line When most folks talk about transmission lines, they generally refer to the line’s characteristic impedance. Getting Started: Characteristic Impedance A proper determination of transmission line impedance is critical for correct impedance matching, especially when we look at newer high frequency analog circuits. Transmission line impedance, particularly even and odd mode impedance, becomes important in a real PCB as transmission lines cannot be considered in isolation.Īs newer products start running at higher frequencies, and with digital devices running at lightning fast switching speeds, more designers need to consider how their traces will act like transmission lines. You have terms like even mode, odd mode, characteristic, differential, common mode.I’m having trouble thinking of another area that uses so many adjectives to describe a single term in one application area. The list of terms used to describe transmission line impedance, including differential lines, is extensive. All transmission lines have some even and odd mode impedance
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